In line differential protection, an electrical power line is protected by two line differential protection relays placed at two distant locations or ends of the power line. By comparing phase information of the current in the power line at the different locations, a differential protection relay may detect critical situations on the power line like short circuits and may isolate the power line from the electrical grid before damage to the electrical equipment will occur.
At each of the different locations of the power line, a measurement device is located that measures the actual current through the power line. The data of each measurement device may then be sent to the differential protection relay, which compares data from different measurement devices, for example, to determine a phase shift between the two currents at two different points of the power line. Here, a problem arises that data measured at the same time needs to be compared, and that a protection relay may therefore need to know the time delays caused by the communication link.
In many cases, electric power utilities use circuit switched SDH (Synchronous Digital Hierarchy) or SONET (Synchronous Optical Network) networks for wide area communication of mission critical operational data like teleprotection signalling or SCADA control and supervision data. Alternatively, data communication between distant locations of an electrical utility may rely on packet switched data networks like Ethernet. However, packet switched data networks may have a variable packet latency, or packet delay variation, for example, the transmission times of packets sent from a first device to a second device may vary over time. Furthermore, the transmission time of a packet sent from a first device to a second device may differ from a transmission time of a packet sent from the second device to a first device. Hence, synchronizing data transmitted over a packet switched network may prove difficult.
An article by W. A. Montgomery entitled “Techniques for Packet Voice Synchronization”, IEEE Journal Vol. SAC-1 No. 6, December 1983, describes ways to transmit packetized voice over a packet switched network and to reconstruct a continuous stream of speech from a set of packets arriving with varying transit delay and even out of order. In long-haul networks, the propagation delay may exceed 100 ms, while significant inter-packet delay variation is caused by packet queuing in the switches. In an embodiment deemed disadvantageous for cost reasons, packet sender and receiver are synchronized to the same absolute time reference, and each packet carries an indication of its production time. For an arriving packet, a target play-out time is determined as a fixed interval after the packet production time. Synchronization of the clocks at the sender and receiver involves a reliable separate channel with known propagation delays.